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ADS1246: die differences with ADS1247 and ADS1248

David GUILLEMOT
Prodigy 90 points
Part Number: ADS1246
Other Parts Discussed in Thread: ADS1247, ADS1248

Hello,

For conception choices, we use 6 * ADS1246 in our system in parallel.

From time to time, with no corelation on a particular system over arround 500 units, it seems than after running fine for few days in a row, an offset of 5 to 10 mV appears.

This offset vanish only with a full restart of the system. (power cycle on/off/on + init of the ADCs registers)

The sensor in front af the ADC has a 200/300 ohm output impedance and is measured trough a diff AINP(~1.5V) - AINN(=1.5V).

We though about other error possibilities, and one of our concern is about a current bias from the ADC excitation current sources to the sensor.

First question :

Is there a difference with the die inside an ADS1246 package regarding the die inside ADS1247/48?

Second question :

If not, is there a possibility that a register IDAC1 badly initiated (normaly not present in ADS1246) can result in a current leakage inside our sensor ?

Thanks for your time and help.

  • 0
    TI__Guru* 94285 points
    Quentin,


    The only case that I've seen where there is a regular offset of about 5-10mV is when the device is set to make a measurement where the AINN input is tied to AVSS (or ground on a unipolar supply). In that case, the input is outside the PGA's range of operation.

    However, I'll answer your questions:

    1. The die for the ADS1246, ADS1247, and ADS1248 is the same. However, we are able to program the functionality for the die. At production, we're able to enable and test different features of the device.

    2. I think it's very unlikely that the IDAC could be initiated and cause a current leakage. Like the other features, by setting the device to be an ADS1246, certain functions are automatically disabled or set. To test this, you could disconnect the sensor excitation and short out your sensor with an ammeter just to see if there is extra current coming out of the input pin. Also you could try to program into the register space normally reserved for ADS1247 and ADS1248, but I've never tried this and I would guess that that section is locked out.

    However, I don't think that IDAC current is the likely problem. Because the error comes up intermittently, I'd look for a different source of error. If you have a schematic, device configuration, and a description of your system and the error, I could review it and see if there's anything that might be a problem.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Thanks for this fast reply.

    As  I was telling, this current leakage is only a  concern among others.

    Our first suspicion is our ADC REF voltage settings wich is 3V-0V=3V > 2V(3V-0V-1V) max from the datasheet.

    In our case, this hypothesis is much harder to test as the hardware is already made. But it is going to be my next task to test that.

    For info, here is our schematic (light and intelless for confidential matters) :

  • 0 in reply to David GUILLEMOT
    TI__Guru* 94285 points

    Quentin,

    Thanks for the basic schematic. There is one thing that I noticed that might be causing a problem. For the ADS1246, the reference input is limited to (AVDD-AVSS)-1. That means if you're using a 3V supply, then you should use a maximum voltage of 2V. The specification is listed in the max column of the Recommended Operating Conditions on page 8 of the datasheet.

    I believe that this limitation is due to the input capacitive sampling charge and discharge of the ADC modulator. During the sampling, charge may be lost with larger references, causing an error.

    I don't think there's a good workaround for this problem. You might try to run the AVDD from the other 3.3V supply and try to drive the sensor from a lower voltage. Alternately you might be able to do a voltage divider from the 3.0V supply to drive the reference.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Yes, this is indeed our first and principal concern.
    But I need some time to try this.
    Keep you in touch in few weeks as I have urgent matter to deal with.